Photoreceptor device module

ABSTRACT

A light receiving element module includes a stem which signal pins penetrate; a base which is fixed in a direction perpendicular to the stem; a cap member which has a light passing-through hole and is fixed to the stem; a spherical lens which is inserted into the light passing-through hole and condenses signal light emitted from the optical fiber; a parabolic mirror which is arranged on the base and reflects the signal light condensed by the spherical lens at approximately a right angle; a light detecting element which is arranged on the base and detects the signal light reflected by the parabolic mirror and converts the signal light into an electrical signal; and a trans-impedance amplifier which is arranged on the base in proximity to the light detecting element and amplifies the electrical signal produced by the light detecting element.

TECHNICAL FIELD

The present invention relates to a light receiving element module onwhich a semiconductor light detecting element such as a photodiode ismounted, and, in particular, to a coaxial type light receiving elementmodule connected to an optical fiber or a light receiving element modulewith an adapter for connection of an optical fiber.

BACKGROUND ART

In recent years, in an optical communication system which transmits anoptical signal via an optical fiber, a speedup of a transmission of anoptical signal is remarkable for responding to increase of communicationtraffic due to population of the Internet, where the transmission speedis being switched from 2.5 Gb/s to 10 Gb/s and research and developmentis being now advanced toward realization of a transmission speed of 40Gb/s. According to such trends, it is required to meet a speeding-upregarding a transmission speed of a signal which is handled by anoptical transmitting/receiving device.

The optical transmitting/receiving device converts a data signal to betransmitted from an electric signal to an optical signal to transmit theoptical signal via an optical fiber for transmission, and receives anoptical signal via an optical fiber for reception to reproduce thereceived optical signal to an electric signal.

As a light receiving element module used in such a kind of lightreceiver, for example, the technique described in Japanese PatentPublication No. 2907203 has been well known. In the patent publication,an optical module has been disclosed which is provided with a box-likehousing which accommodates a light receiving element and has a mountingface on which the light receiving element is mounted, a sleeve whichextends from a side wall of the box-like housing in a predetermineddirection and supports a ferrule mounted at a distal end of an opticalfiber in a state that the ferrule has been accommodated therein, an ovalface reflecting mirror which is accommodated in the box-like housing forcoupling the optical fiber and the light receiving element optically,and a supporting structure for holding the oval face reflecting mirrorat a predetermined position inside the box-like housing in a state thatthe oval face reflecting mirror has been separated from the mountingface in the box-like housing by a predetermined distance.

in the technique described in the patent publication, however, since theoval face reflecting mirror is used, it is necessary to elongate a focallength of the reflecting mirror in order to arrange the optical fiberand the light receiving element optimally, which causes such a problemthat the light receiving element module can not be reduced in size.Further, since a space between a reflecting point of the reflectingmirror and the light receiving element is large and the focal length islong, a thermal expansion of the material for the reflecting mirroroccurs due to an environmental temperature. As a result, there is aproblem that since change of an image point position becomes large, acompensation unit for the image point position illustrated in theJapanese Patent Publication No. 2907203 is required for preventing thechange, which results in complication in structure. Furthermore, sincethe shape of the reflecting mirror has the oval face, there occurs sucha problem that it is necessary to use a mirror-finishing milling machinefor manufacturing a forming mold for a mirror face used in a plasticmold and it is difficult to secure a face accuracy.

Accordingly, an object of the present invention is to provide a lightreceiving element module with a simple structure, which does not requirea complicated structure such as a temperature compensation unit for animage point position, and which can be reduced in size.

DISCLOSURE OF THE INVENTION

A light receiving element module according to the invention detectssignal light emitted from an optical fiber and includes a lens whichcondenses signal light emitted from the optical fiber; a reflectingmirror which has a quadric surface which reflects the signal lightcondensed by the lens; and a light detecting element which detects thesignal light reflected by the reflecting mirror and converts the signallight into an electrical signal.

The reflecting mirror may be a parabolic mirror.

The signal light condensed by the lens may be incident on the reflectingsurface generally in parallel with the axis of the reflecting surface,and the signal light which is incident on a position offset from thecenter of the reflecting mirror by approximately a radius may bereflected on the reflecting surface.

The signal light condensed by the lens may be incident on the reflectingsurface generally in parallel with the axis of the reflecting surface,and the signal light incident may be reflected at an approximately rightangle on the reflecting surface.

The reflecting mirror may be a hyperboloid mirror.

The lens may be a spherical lens.

The light receiving element module may include a trans-impedanceamplifier which is arranged on the same flat face as the light detectingelement in proximity to the light detecting element and amplifies theelectrical signal converted by the light detecting element.

The reflecting mirror may be a member which is formed by using a plasticmold and on which a reflecting surface is provided.

Adjustment of the optical axis of the optical fiber in three axialdirections of the optical axis direction and two directionsperpendicular to the optical axis with respect to an optical axisprovided by the optical fiber and the lens, may be performed.

A light receiving element module according to the invention detectssignal light emitted from an optical fiber, and includes a stem whichsignal pins penetrate; a base which is fixed in a directionperpendicular to the stem; a cap member which has a lightpassing-through hole and is fixed to the stem; a spherical lens which isinserted into the light passing-through hole and condenses signal lightemitted from the optical fiber; a parabolic mirror which is arranged onthe base and reflects the signal light condensed by the spherical lensat approximately a right angle; a light detecting element which isarranged on the base and detects the signal light reflected by theparabolic mirror and converts the signal light to an electrical signal;and a trans-impedance amplifier which is arranged on the base inproximity to the light detecting element and amplifies the electricalsignal produced by the light detecting element.

A light receiving element module according to the invention detectssignal light emitted from an optical fiber, includes a stem which signalpins penetrate; a base which is fixed in a direction perpendicular tothe stem; a cap member which has a first light passing-through hole andis fixed to the stem; a window member which covers the first lightpassing-through hole; a lens holding member which has a second lightpassing-through hole and is fixed to the cap member; a spherical lenswhich is inserted into the second light passing-through hole andcondenses signal light emitted from the optical fiber; a parabolicmirror which is arranged on the base and reflects the signal lightcondensed by the spherical lens at approximately a right angle; a lightdetecting element which is arranged on the base and detects the signallight reflected by the parabolic mirror and converts the signal light toan electrical signal; and a trans-impedance amplifier which is arrangedon the base in proximity to the light detecting element and amplifiesthe electrical signal produced by the light detecting element.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an appearance constitution of a light receivingelement module according to a first embodiment;

FIG. 2 schematically illustrates a vertical sectional view of the lightreceiving element module in FIG. 1;

FIG. 3 is a diagram for explaining spreading of a Gaussian beam;

FIG. 4 is a diagram for explaining various symbols;

FIG. 5 is a diagram for explaining various symbols;

FIGS. 6A and 6B are diagrams for explaining relationship between spacebetween an object point and lens and lateral magnification;

FIGS. 7A and 7B are diagrams for explaining relationship between spacebetween an object point and a lens and distance between an R point andan image point;

FIGS. 8A and 8B are a horizontal sectional view and a vertical sectionalview, respectively, of the light receiving element module in FIG. 1;

FIGS. 9A and 9B are longitudinal and sectional views, respectively, ofthe light receiving element module of FIG. 1;

FIGS. 10A, 10B, and 10C are, respectively, a vertical sectional view ofthe periphery of a parabolic mirror of the light receiving elementmodule, a front view of the mirror, and a plan view of the lightreceiving element module with the mirror removed;

FIGS. 11A, 11B, and 11C are, respectively, a vertical sectional view ofthe periphery of a parabolic mirror of the light receiving elementmodule, a front view of the mirror, and a plan view of the lightreceiving element module with the mirror removed;

FIGS. 12A, 12B, and 12C are, respectively, a vertical sectional view ofthe periphery of a parabolic mirror of the light receiving elementmodule, a front view of the mirror, and a plan view of the lightreceiving element module with the mirror removed;

FIG. 13 explains a light receiving element module of a third embodiment;and

FIG. 14 explains a light receiving element module of a fourthembodiment.

BEST MODE FOR CARRYING OUT THE INVENTION

Exemplary embodiments of a light receiving element module according tothe present invention are described below with reference to theaccompanying drawings.

First Embodiment

With reference to FIG. 1 to FIGS. 11A, 11B, and 11C, a light receivingelement module of the first embodiment of this invention will beexplained. The light receiving element module of this first embodimenthas the module aspect of an inexpensive can-package type, and aphotodiode is housed in a package as a light detecting element. Further,in the description, the light receiving element module is a generic namegiven to modules including a light receiving element module which doesnot have a cap (a lid) for sealing.

FIG. 1 illustrates an appearance constitution of a light receivingelement module 3. As illustrated in FIG. 1, the light receiving elementmodule 3 includes a can-package 1 provided with a cap member 13 and astem 10, and a receptacle 2 in which a ferrule 21 connected with anoptical fiber 20 is inserted. The stem 10 generally has a diameter of 6millimeters or less.

FIG. 2 schematically illustrates a vertical sectional view of the lightreceiving element module 3 for explaining the light detecting principleof the light receiving element module 3 of FIG. 1. FIG. 2 illustratesthe structure of FIGS. 8A and 8B described later in a simplified manner,where illustration of some portions is omitted and some portions areillustrated in simplified manner.

As illustrated in FIG. 2, the light receiving element module 3 includesa stem 10 which a signal pin 40 (corresponds to signal pins 41 a and 41b, ground pins 42 a and 42 b, and voltage supplying pins 43 a and 43 b)penetrates, a base 11 fixed to the stem 10 in a direction perpendicularthereto, a cap member 13 which has a light passing-through hole 14 andis fixed to the stem 10, and a spherical lens 12 which condenses signallight emitted from the optical fiber 20. Further, the light receivingelement module includes a parabolic mirror 16 which is disposed on thebase 11 and reflects the signal light condensed by the spherical lens 12approximately at a right angle, a light receiving element 18 which isdisposed on the base 11 and receives the signal light reflected by theparabolic mirror 16 to convert the same to an electric signal, atrans-impedance amplifier 19 which is disposed on the base 11 inproximity to the light receiving element 18 and amplifies the electricsignal converted by the light receiving element 18 and the like. Withthe light receiving element 18, a photodiode is used in this embodiment.

The signal pin 40 penetrates the stem 10 via a dielectric 607(corresponding to dielectrics 61, 63 a, and 63 b), and the base 11 andthe cap member 13 are fixed to the stem 10 in a direction perpendicularthereto. The light receiving element 18, the parabolic mirror 16 and thetrans-impedance amplifier 19 are put on the base 11 in proximity to oneanother. The light passing-through hole 14 for inserting the sphericallens 12 is formed in the cap member 13, and the spherical lens 12 isinserted into the light passing-through hole 14 of the cap member 13 sothat a sealed structure of the interior of the cap member can beachieved. The spherical lens 12 can be constituted by, for example, aninexpensive BK7 (having a reflection index of 1.51: Trade Name of ShotInc.). Further, the receptacle 2 formed with an insertion hole 22 forinserting the ferrule 21 is fixed to the cap member 13. The parabolicmirror 16 has a reflecting surface 16 a, and it is arranged such thatthe signal light condensed by the spherical lens 12 is incident on aportion of the parabolic face (the reflecting surface) which is offsetfrom a rotation symmetry axis at a distance corresponding to about aradius. Incidentally, the radius used here means a radius of curvatureof the parabolic face described later.

A transmission route of signal light emitted from the optical fiber 20will be explained next. Signal light emitted from the optical fiber 20is incident on the spherical lens 12. The spherical lens 12 condensesincident signal light. A principal ray of the signal light condensed bythe spherical lens 12 is incident on the reflecting surface 16 agenerally in parallel to the rotation symmetry axis of the reflectingsurface 16 a of the parabolic mirror 16. The incident signal light isreflected generally at a right angle on the reflecting surface 16 a ofthe parabolic mirror 16 to be incident on the light receiving element18. The signal light is condensed by the reflection due to thecharacteristic of the parabolic mirror 16. The light receiving element18 converts incident signal light to an electric signal to output thesame to the trans-impedance amplifier 19. In the trans-impedanceamplifier 19, the electric signal from the light receiving element 18 isamplified, is outputted through the signal pin 40 to an external uppersystem. Thus, according to the light receiving element module of thepresent invention, two-stage condensation is performed at the sphericallens 12 and the parabolic mirror 16.

The arrangement of the optical fiber 20, the spherical lens 12, theparabolic mirror 16, and the light detecting element 18 will beexplained briefly. A virtual image of a light detecting face (a photodetector, (hereinafter, “PD”) light detecting face) of a light draftingelement is located on an optical axis of signal light emitted from theoptical fiber. On the other hand, a portion (hereinafter, “an emittingpoint”) of the optical fiber from which a signal light is emitted isarranged on an object point, and a real image is imaged at the imagingpoint of the optical fiber on the optical axis of the signal light bythe spherical lens. At this time, the optical fiber 20, the sphericallens 12, the parabolic mirror 16 and the light receiving element 18 arearranged such that the position of the real image of the emitting pointof the optical fiber is imaged on the position of the virtual image ofthe light detecting face. That is, the virtual image is formed on thelight detecting face of the light detecting element by the reflectingmirror on the optical axis of the lens, and the lens transfers the lightemitting point of the optical fiber placed at the object point to thevirtual image plane of the light detecting face of the light detectingelement. In other words, such a constitution can be employed that afiber image is formed on the optical axis of the lens, and the fiberimage is transformed to the light detecting face of the light detectingelement by the parabolic mirror.

A specific design example of the spherical lens 12 and the parabolicmirror 16 will be explained next with reference to FIG. 3 to FIGS. 7Aand 7B. The magnification of the spherical lens 12 will be firstexplained using FIG. 3 to FIG. 5. FIG. 3 is a diagram for explainingspreading of a Gaussian beam, and FIG. 4 and FIG. 5 are diagrams forexplaining various symbols.

Supposing an ideal lens, a spot radius ω2 of an image, where an opticalfiber with a spot radius ω1 which is located on an object point andemits light with a wavelength λ, is on an image plane defocused from aparaxial image point by a distance z via an ideal lens of a lateralmagnification m, can be expressed by the following equation (1).

$\begin{matrix}{{\omega_{2}(z)} = {m\;\omega_{1}\sqrt{1 + \left( \frac{\lambda\; z}{{\pi\left( {m\;\omega_{1}} \right)}^{2}} \right)^{2}}}} & (1)\end{matrix}$

FIG. 3 illustrates spot radiuses on an image plane between the paraxialimage point and the defocus from 0 to 60 micrometers obtained via anideal lens with a lateral magnification of 0.5 to 1 for each 10micrometers regarding an optical fiber with a wavelength of 1.3micrometers and a spot radius of 5 micrometers utilizing the equation(1). Since the Gaussian beam outside about 1.5 times a spot radius (alight intensity of 1/e²) causes loss of about 2%, a spot radius of 7.5micrometers or less to the light receiving element with a radius of 10micrometers used for light transmission of 10 Gb/s satisfies a suitablecondition. Incidentally, introductory remarks in FIG. 3 mean a lateralmagnification and a defocus amount (millimeter). That is, for example,ω2 (m, 60×10⁻³) means a spot radius (ω2) obtained when the lateralmagnification is m times and the defocus amount is 60 micrometers.

When considering, for example, a thickness tolerance of ±30 micrometersfrom FIG. 3, it is understood that the lateral magnification of 0.7 isoptimal. However, a lateral magnification of about 1 is excellent in anoptical system having a large optical axis shift (for example, of thedefocus amount of 60 micrometers) and so on. Practically, considering animage blur due to the aberration of the optical system and an assemblingtolerance, the lateral magnification of the whole optical system is setto a range of 0.5 to 1.

The optical system including the lens 12 and the parabolic mirror 16will be explained next with reference to FIG. 4. In FIG. 4, it isassumed that the lens 12 is an ideal lens 120 with a focal length f1 andthe parabolic mirror 16 has a reflecting surface 16 a constituting aparaboloid 16 with a radius of curvature r in the vicinity of a rotationsymmetry axis z. The parabolic face 16 is a paraboloid of z=y²/2 r to anaxis y perpendicular to the optical axis, and a principal ray from thelens is incident on a position (a point R) of the height h from therotation symmetry axis and is reflected. An inclination of a principalray generated in a manner shifted from the optical axis by δ is definedas u, a crossing point between a ray reflected at a time of u=0 and therotation symmetry axis is defined as a point Q, an angle formed betweenthe reflected beam and y axis is defined as θ, and the position wherethe point Q is shifted by δ′ at a time of angle u is defined as Q′.According to the paraxial and coaxial optical system, it is assumed thatthe magnification is defined as γ=δ′/δ in the optical system of FIG. 4.Since δ=f₁ tan u,

${{RQ}^{\prime} = \frac{h}{\cos\;\theta}},$and

$\delta = {{{RQ}^{\prime} \cdot \frac{\sin\mspace{11mu} u}{\cos\mspace{11mu}\theta}} = \frac{h\mspace{11mu}\sin\mspace{11mu} u}{\cos^{2}\theta}}$is obtained from FIG. 4, the magnification is obtained by the followingequation.

$\begin{matrix}{\gamma = {{\frac{h}{f_{1}}\frac{\sin\mspace{11mu} u}{\tan\mspace{11mu} u}\frac{1}{\cos^{2}\theta}} \approx {\frac{h}{f_{1}}\frac{1}{\cos^{2}\theta}}}} & (2)\end{matrix}$Since the paraboloid is

${z = \frac{y^{2}}{2\; r}},$the angle θ can be obtained from

${\tan\mspace{11mu}\theta} = {\frac{\frac{r}{2} - \frac{h^{2}}{2\; r}}{h}.}$

According to the equation (2), it will be understood that themagnification y is influenced by the position h on the paraboloid 16upon the principal ray is incident. When such a usage is applied, theparabolic mirror 16 may be used in the vicinity of h≈r for reducingaberration, and it can be thought that the focal length is f2=r, and aprincipal plane is a plane including R point at which the principal rayis incident upon the reflecting mirror.

A partial system of an optical system constituting the parabolic mirror16 is explained with reference to FIG. 5. Considering that the parabolicmirror 16 is an ideal lens 162 with a focal length f2, and assuming thata spot radius formed on a light detecting face of the light detectingelement is ωpd, a space between the light detecting face and theprincipal plane is d0, a spot radius of a virtual image is ωpd′, and thedistance between the virtual image and the principal plane is d1 (whichis a virtual image and takes a negative number), the lateralmagnification m2 of the partial system and d1 can be expressed by thefollowing equations.

$\begin{matrix}{{m2} = {\frac{\omega_{pd}}{{\omega_{pd}}^{\prime}} = \sqrt{{\left( \frac{\pi\;\omega_{pd}^{2}}{\lambda} \right)^{2}\left( \frac{1}{f_{2}} \right)^{2}} + \left( {1 - \frac{d_{0}}{f_{2}}} \right)^{2}}}} & (3) \\{d_{1} = \frac{{\left( \frac{\pi\;\omega_{pd}^{2}}{\lambda} \right)^{2}\left( \frac{1}{f_{2}} \right)} - {d_{0}\left( {1 - \frac{d_{0}}{f_{2}}} \right)}}{{\left( \frac{{\pi\omega}_{pd}^{2}}{\lambda} \right)^{2}\left( \frac{1}{f_{2}} \right)^{2}} + \left( {1 - \frac{d_{0}}{f_{2}}} \right)^{2}}} & (4)\end{matrix}$

FIG. 6A illustrates a lateral magnification m2 of the partial systemcorresponding to the distance d0 showing the space between the principalplane and the light detecting face (PD light detecting face) 18 a of thelight detecting element 18 regarding the parabolic mirror 16 having aparabolic face with a radius r varying from 0.55 millimeters to 0.95millimeters. Further, FIG. 6B illustrates the virtual image position d1versus the distance d0 regarding the reflecting mirror having theparaboloid 16 with the radius r varying from 0.55 millimeter to 0.95millimeter. Incidentally, introductory remarks in respective diagrams ofFIGS. 6A and 6B mean the radius (millimeters) of the paraboloid of theparabolic mirror 16 and the space (millimeters) between the principalplane and the light detecting face (PD light detecting face) 18 a of thelight detecting element 18. That is, for example, m2 (0.55, d0) meansthe lateral magnification (m2) when the radius is 0.55 millimeter andthe space between the principal plane and the PD light receiving face isd0.

In a simplified example, assuming that the lateral magnification m ofthe entire optical system is one and the lateral magnification m2 of thepartial system obtained by the reflecting mirror of the parabolic face16 is 0.5, the distance between the point R (principal plane) and the PDlight detecting face 18 a varies from 0.28 millimeter to 0.48 millimeteraccording to variation of the radius r of the reflecting mirror havingthe paraboloid 16 from 0.55 millimeter to 0.95 millimeter, which issuitable for maintaining the height of the rising portion of a wire bondused for wiring of the light detecting element 18, so that the wire bondand the reflecting mirror face of the parabolic mirror 16 do not contacteach other.

A partial system of the spherical lens 12 will be explained next. Afirst lens on which light emitted from the optical fiber 20 is incidentis a spherical lens 12 with a radius R and a refractive index n, and itsfocal length is f1=R/(2(n−1)). A ray trace in FIG. 2 illustrates adiagram where 11 rays have been traced in a range of NA 0.2 for each NA0.04, where a ray with NA 0.16 and a ray with NA 0.2 cross. In thismanner, though the spherical lens 12 is inexpensive, its sphericalaberration is large and aberration increasing distribution of a lenspower (a refractive power) increases, so that the aberration issuppressed by setting the lateral magnification of the partial system ofthe lens to two times and distributing power to the parabolic mirror 16.In this connection, since NA of the optical fiber for the wavelength of1.3 micrometers and the spot radius of 5 micrometers is 0.1 at anintensity of 1/e² and a light intensity distribution of about 98% can beachieved at NA of 0.15, blur of an image becomes very small with such anaberration distribution.

Incidentally, for example, assuming the reflecting mirror of theparabolic mirror 16 is a flat mirror and aberration is achieved by anfinite system including one lens, even assuming a preferable lateralmagnification is about 0.8 to 0.9 or so, a shading loss to a lightreceiving radius of 10 micrometers becomes large as about 5%. Therefore,the lateral magnification of the partial system of the lens ispreferably designed one time or more.

The constitution of the parabolic reflecting mirror will be explainednext. A hyperbolic reflecting mirror can achieve an aplanatic conditionin the optical system illustrated in FIG. 2, and a parabolic reflectingmirror (the parabolic mirror 16) can achieve an aplanatic condition whenit converges a collimated beam. However, the parabolic reflecting mirrorhas a rotation symmetry axis that is parallel to the optical axis sothat forming molds can be manufactured by a mirror finishing lathe witha high cutting accuracy instead of a mirror finishing milling machinefor manufacturing an oval surface or a hyperboloid, and assembling ofthe forming molds can be easy.

A coefficient of thermal expansion α of plastic suitable formanufacturing a parabolic reflecting mirror (the parabolic mirror 16) is5.6×10⁻⁵. For example, a movement of the reflecting point (R point) froma bottom of the reflecting mirror is about 2.5 micrometers when aparabolic mirror with a radius r of 0.85 millimeters is used withlateral magnification of 0.5 and the temperature varies from 25° C. to85° C., assuming that a space between a reflecting point (R point inFIG. 4) and a light receiving face (the PD light receiving face) 18 a ofthe light receiving element 18 is 450 micrometers, the thickness of alight receiving element 18 is 150 micrometers, and the thickness of asubstrate (a chip carrier) for a light receiving element 17 made ofceramic is 145 micrometers. On the other hand, focal point change issmall because of the longitudinal magnification of 0.25, though thefocal length change of the parabolic mirror is 2.8 micrometers.Similarly, when the radius is 1 millimeter or less and the lateralmagnification of the partial system of the parabolic reflecting mirroris one time or less, blur of an image due to a temperature change of theimage point is reduced, and a suitable optical system can be obtainedwith a simple structure without arranging the image point compensationunit with a complicated structure described in Japanese PatentPublication No. 2907203.

The merits of the optical system are further explained with reference toFIG. 7A and FIG. 7B. As illustrated in FIG. 2, the lens 12 is mounted inthe cap 13, and the cap 13 is welded on the stem 10 to produce anair-tight structure, by a method such as projection welding. However, itis relatively difficult to secure position accuracy in the welding step.For example illustrated in FIG. 7A, if misalignment Δ between thereflecting point R of the parabolic mirror 16 and the central axis ofthe lens 12 takes place when welding, a decrease in the light detectionsensitivity about 2% occurs, when the misalignment Δ is 100 micrometers,as shown in FIG. 7B, due to the optical fiber 20 alignment Δ/ml from thelens center 12. Also, proper adjustment g between the lens principalplane and the fiber 20, where m1 is lateral magnification of the partialsystem of the lens, is required. With such a constitution, when theoptical fiber 20 or the receptacle 2 is adjusted properly in the opticalaxial direction and a direction perpendicular to the optical axis,misalignment of respective parts are compensated for, so that a suitableoptical coupling can be obtained.

A detailed constitution of the light receiving element module 3 of FIG.1 will be explained next. FIGS. 8A and 8B illustrate a horizontalsectional view and a vertical sectional view of the light receivingelement module 3 of FIG. 1. As illustrated in FIGS. 8A and 8B, the lightreceiving element module 3 is provided with a disc-like stem 10 mountedwith signal pins 41 a and 41 b that constitute a differential feed, asupplying pin 43 a for a bias voltage to a photodiode 18, a supplyingpin 43 b for a power source voltage to the trans-impedance amplifier 19,ground pins 42 a and 42 b and the like, a trapezoidal column-shape base11 mounted with a parabolic mirror 16 and a plurality of elements, aspherical lens 12 for condensing signal light emitted from the opticalfiber 20, a cylindrical cap member 13 for sealing the base 11 and thelike from the outside, a receptacle 2 in which a ferrule 21 connectedwith the optical fiber 20 is inserted, and the like.

The cap member 13 exhibits a two-stage cylindrical shape so as toinclude a first cap member 13 a fixed to the stem 10 by projectionwelding or the like and a second cap member 13 b fitted on a distal endof the first cap member 13 a and fixed to the first cap member 13 a byYAG welding or the like.

A light passing-through hole 14 for insertion of the spherical lens 12is formed at a distal end of the first cap member 13 a, and thespherical lens 12 is inserted into the light passing-through hole 14.The spherical lens 12 is constituted with, for example, BK7 (arefractive index of 1.51), and it is fixed to the first cap member 13 aby solder glass with a low melting point. An inner space 15 of the firstcap member 13 a is isolated by the spherical lens 12 from the outside,so that the inner space 15 in which the base 11 is accommodated ismaintained in an air-tight state.

By positioning and adjusting the second cap member 13 b in a directionin which the ferule 21 (refer to FIG. 2) is inserted (in an opticalaxial direction) to fix the same to the first cap member 13 a, alignmentbetween the spherical lens 12 and the optical fiber 20 inserted into thereceptacle 2 in the optical axial direction is performed.

The receptacle 2 has a ferule insertion hole 22 in which the ferule 21connected with the optical fiber 20 and a light passing-through window23 for allowing passing-through of signal light emitted from the opticalfiber 20. The receptacle 2 is fixed to the second cap member 13 b by YAGwelding or the like. When the receptacle 2 is fixed to the second capmember 13 b, positioning between the spherical lens 12 and the opticalfiber 20 mounted to the receptacle 2 regarding two directionsperpendicular to the optical axis is aligned by conducting positioningand adjusting in the two directions perpendicular to the optical axis.Thus, when the second cap member 13 b and the receptacle 2 are fixed,they are positioned and adjusted, so that adjustment in three axialdirections to the optical axis is performed.

The ferrule 21 connected with the optical fiber 20 has a propermechanism (not illustrated) for, when the ferrule 21 is inserted intothe ferrule insertion hole 22 of the receptacle 2, pressing the ferrule21 to lock and fix the ferrule 21 to the receptacle 2.

The constitution of the interior of the can package 1 will be explainednext. FIGS. 9A and 9B illustrate arrangement relationship among the stem10, the pins and the base 11. As illustrated in FIGS. 9A and 9B, the canpackage 1 is constituted with a disc-like stem 10 with a plurality ofpins and a trapezoidal column-like base 11 fixed to an inner wall faceof the stem 10 in a direction perpendicular thereto by Ag brazing or thelike.

The stem 10 constituting a ground is mounted with a pair of signal pins41 a and 41 b constituted differential feed for signal transmission ofthe light receiving element 18, two ground pins 42 a and 42 b placed onboth sides of the signal pins 41 a and 41 b, and voltage supplying pins43 a and 43 b for supplying a power source voltage of thetrans-impedance amplifier 19 and supplying a bias voltage to the lightreceiving element 18.

The signal pins 41 a and 41 b and the ground pins 42 a and 42 bconstitute a field-through which penetrates the stem 10. Theserespective signal pins are fixed to the stem 10 in an air-tight sealingstate via dielectrics (61, 63 a, and 63 b) formed from material such asglass. The ground pins 42 a and 42 b are fixed to an outer wall face 10z of the stem 10 constituting the ground by pressure-fitting andwelding.

In further detailed explanation, the stem 10 is formed of metals such askovar (Fe—Ni alloy), soft iron or CuW (copper tungsten), and plating ofNi, gold or the like is ordinarily applied to an upper layer of thestem. Further, for example, in the case of kovar (Fe—Ni alloy) or softiron, the stem 10 can be manufactured by punching out a metal platethereof with a die. For example, in the case of CuW, the stem can bemanufactured using a metal injection molding technique, and themanufacturing cost is low because of the process is simple. The stem 10includes a plurality of holes 51, 53 a, and 53 b in a dispersed manner,and dielectrics 61, 63 a, and 63 b are respectively inserted into theseholes 51, 53 a, and 53 b.

A pair of pin insertion holes (reference numerals are omitted) areformed in the dielectric 61, and the signal pins 41 a and 41 b areinserted and fixed in these pin insertion holes. Similarly, holes(reference numerals are omitted) are respectively formed in thedielectrics 63 a and 63 b, and voltage supplying pins 43 a and 43 b areinserted and fixed in the respective holes. The shape of the dielectric61 in which the pair of signal pins 41 a and 41 b are inserted is anelliptic shape in this case. Correspondingly, the hole 51 in which thedielectric 61 is inserted is also an elliptic shape. The otherdielectrics 63 a and 63 b are formed in a circular shape. Incidentally,the ground pins 42 a and 42 b do not penetrate the stem and are fixed tothe outer wall face 10 z of the stem 10 by pressure-fitting and welding,as described above.

As the dielectrics 61, 63 a, and 63 b, for example, kovar glass (sodabarium glass), boro-silicated glass, or the like is used. Further, asthe signal pins 41 a and 41 b, the voltage supplying pins 43 a and 43 b,and the ground pins 42 a and 42 b, for example, a metal such as kovar,50% Ni—Fe alloy, or the like is used.

When the stem 10 and the base 11 are manufactured as separate membersfrom each other, the base 11 is connected and fixed to the stem 10 by Agbrazing or the like. Of course, the stem 10 and the base 11 may bemanufactured as an integral member.

A differential line substrate 31, a trans-impedance amplifier circuitelement 33, a light receiving element circuit element 32, atrans-impedance amplifier 19, a parabolic mirror 16, and a lightreceiving element substrate 17 are mounted on an upper face of the base11. When a capacitor of a ceramic chip type is used as the substrate 17for a light receiving element, connection with the light receivingelement is achieved by soldering fixation so that inductance can bereduced and resonance due to wiring with the trans-impedance amplifier19 or the like can be prevented, which is preferable. The base 11constitutes a whole ground conductor layer (hereinafter, “solid ground”)by plating, and the solid ground is connected to a ground formed on aback face of each element as flat conductor plates.

The differential line substrate 31 includes a pair of differentialmicrostrip lines 31 a and 31 b formed symmetrically, and a solid ground(not illustrated) formed on a back face of the substrate. One ends ofthe differential microstrip lines 31 a and 31 b are formed with a pairof pads (91 a and 91 b), wider portions 92, and stubs 93 a and 93 b fortaking impedance matching with a field-through portion and front sideand rear side circuits. End portions of the signal pins 41 a and 41 bmounted on the stem 10 are connected and fixed to the pads of thedifferential line substrates 31 by brazing or soldering.

The light receiving element circuit element 32 is mounted with a circuitelement (a capacitor, a resistance, a coil or the like) for eliminatingnoises in a certain frequency band when a bias voltage is applied to thelight receiving element 18 mounted on the substrate 17 for a lightreceiving element. The light receiving element circuit element 32 isformed with a plurality of pads (reference numerals are omitted), it isconnected to the voltage supplying pin 43 a via a wire bond 95 a and isconnected to a pad of the light receiving element substrate 17 viaanother wire bond 70 c.

A trans-impedance amplifier circuit element 33 is mounted with a circuitelement (a capacitor, a resistance, a coil or the like) for eliminatingnoises in a certain frequency band of a power source voltage supplied tothe trans-impedance amplifier 19. The trans-impedance amplifier circuitelement 33 is formed with a plurality of pads (reference numerals areomitted), and it is connected to the voltage supplying pin 43 b via awire bond 95 b and is connected to the pad of the trans-impedanceamplifier 19 via a wire bond 70 d.

An output terminal for a differential signal from the trans-impedanceamplifier 19 is connected to pads of the differential line substrate 31and the like via wire bonds 96 a and 96 b. Further, the trans-impedanceamplifier 19 is connected to pads of the light detecting element 18, thelight detecting element circuit element 32 and the like via wire bonds(which will be described later regarding FIGS. 10A, 10B, and 10C). Thetrans-impedance amplifier 19 converts current/voltage of an electricalsignal received from the light detecting element 18 and amplifies theelectrical signal.

The light receiving element substrate 17 is mounted with a lightreceiving element 18 such as, for example, a pin-type photodiode, and itis formed with a plurality of pads (reference numerals are omitted) andis connected to the light receiving element circuit element 32 and thetrans-impedance amplifier 19 via wire bonds. The light receiving element18 receives signal light reflected by the parabolic mirror 16 to convertit to an electric signal (a monitor signal). After the electric signalis amplified by the trans-impedance amplifier 19, the amplified signalis outputted from output terminals for a differential signal in thetrans-impedance amplifier 19, and outputted from the signal pins 41 aand 41 b mounted on the stem 10 via a pair of differential data lines 31a and 31 b of the differential line substrate 31 to an upper system.

The parabolic mirror 16 is formed in a plastic mold. As illustrated inFIGS. 8A and 8B, the parabolic mirror 16 has a reflecting surface 16 ain the shape of a paraboloid, and includes a groove (refer to FIGS. 10A,10B, and 10C) for connecting the light detecting element 18 and thetrans-impedance amplifier 19 via a wire bond. The reflecting surface 16a is given with a base film with an excellent adhesion such as chromiumand then a metal film such as gold, aluminum, silver with a highreflectivity is applied using a method such as electron beam vapordeposition or sputtering. Further, the reflecting film may be one wheredielectric film with multiple layers of titanium dioxide or silicondioxide, or alumina or tantalum pentoxide has been used, or it may beone where a protective film of dielectric has been applied on a metalfilm. Incidentally, an effect for prevention of short-circuiting with awire bond can be achieved by applying an insulating film on a surface ofthe reflecting surface 16 a, which is preferable.

The reflecting surface 16 a of the parabolic mirror 16 reflects signallight condensed by the spherical lens 12 at an angle of about 90° so thesignal light reaches the light detecting face 18 a of the lightdetecting element 18, and the reflecting surface 16 a has a parabolicshape, so that aberrations are hardly generated and the responsivity ofthe light detecting element 18 can be increased.

By reflecting a raypath of signal light at almost a right angle by theparabolic mirror 16 in this manner, it is made possible to arrange thespherical lens 12 and various electric parts at a position horizontaldirection to the surface of the light receiving element 18, and it ismade possible to reduce the thickness of the light receiving elementmodule.

FIGS. 10A, 10B, and 10C are diagrams for explaining electricalconnection of the light detecting element 18 and the trans-impedanceamplifier 19, wherein FIG. 10A is a vertical sectional view of theperiphery of the parabolic mirror 16, FIG. 10B is a front view, and FIG.10C is a plan view when the parabolic mirror 16 has been removed. Asillustrated in FIGS. 10A to 10C, the light detecting element 18 and thetrans-impedance amplifier 19 are mounted on one flat face of the base 11in proximity to each other. The light detecting element 18 mounted onthe light receiving element substrate 17 includes a photodiode of asurface incident type having a light detecting face on a surface side,and a light detecting face (a photodiode portion) 18 a and a pad 18 b(for example, a p-side electrode) which is an electrode on the surfaceside. Further, an electrode (for example, an n-side electrode) islocated on the side of the light receiving element substrate 17.

A groove 16 b for connecting the light detecting element 18 and thetrans-impedance amplifier 19 by a wire bond is formed on the parabolicmirror 16. Incidentally, the groove 16 b has a semi-cylindrical shape inthe drawing, but it is not limited to this shape. For example, thegroove may have a rectangular parallelepiped shape. That is, if thegroove penetrates the parabolic mirror 16 like a tunnel in a state thatthe parabolic mirror 16 has been mounted on the base 11, it can take anyshape. A pad 19 b for inputting an electric signal and a ground 19 a areformed on the trans-impedance amplifier 19. A pad 18 b on an anode sideof the light detecting element 18 and the pad 19 b of thetrans-impedance amplifier 19 are respectively bonded to one end and theother end of a wire bond 70 b. An electrode (not illustrated) on acathode side of the light detecting element 18 is soldered an electrode17 a of the light receiving element substrate 17. An electrode 17 a ofthe light receiving element substrate 17 is connected to a lightreceiving element circuit element 32 via a wire bond 17 c, and the lightreceiving element circuit element 32 is connected to a voltage pin 43 a.An electrode on a back face of a capacitor 32 b is connected to theelectrode 17 a of the light receiving element substrate 17. An electrodeon a surface of the capacitor 32 b is connected to a ground face 17 b ofthe light receiving element substrate 17 via a wire bond 70 e. Further,an electrode on a surface of the capacitor 32 b is connected to theground 19 a of the trans-impedance amplifier 19 via a wire bond 70 a.The ground face 17 b of the light receiving element substrate 17 isconnected to a surface (a ground face) of the base 11 via a through hole17 c.

FIGS. 11A to 11C are diagrams for explaining electrical connection ofthe light detecting element 18 and the trans-impedance amplifier 19,wherein, as another example. FIGS. 10A to 10C, FIG. 11A is a verticalsectional view of the periphery of the parabolic mirror 16, FIG. 11B isa front view and FIG. 11C is a plan view when the parabolic mirror 16has been removed. As illustrated in FIGS. 11A to 11C, the structure maybe simplified by using a parallel flat-plate capacitor 170 of a ceramicchip type instead of the light receiving element substrate 17. In thiscase, a back face of the light detecting element 18 is mounted on anupper face of the capacitor 170 of a ceramic chip type and the back faceof the capacitor 170 of a ceramic chip type is connected to a groundface of the base 11. That is, flat faces of electrodes at both ends ofthe capacitor 170 are made parallel, fixation is made such that theelectrode at a lower end of the capacitor 170 is electrically connectedto the ground face of the base 11, and placement is made such that theelectrode at an upper end of the capacitor 170 is electrically connectedto the electrode at a back face side of the photodiode. Further, theground 19 a of the trans-impedance amplifier 19 is connected to theground face of the base 11 in the same manner as. in FIGS. 10A to 10C.Furthermore, the pad 19 b of the trans-impedance amplifier 19 isconnected to the pad 18 b of the light detecting element 18.

According to the light receiving element module of the first embodiment,since such a constitution is employed that signal light emitted from theoptical fiber 20 is condensed by the spherical lens 12 and the condensedsignal light is reflected by the parabolic mirror 16, the region of thereflecting surface 16 a of the parabolic mirror 16 can be made small,and the parabolic mirror 16 can be reduced in size. Thereby, it is madepossible to reduce the light receiving element module in size.Furthermore, influence of a thermal expansion coefficient due tomaterial for the reflecting mirror is reduced and structure issimplified.

Further, since the parabolic mirror 16 is constituted so as to reflectsignal light to an approximately right angle at a position approximatelyoffset from the center of the paraboloid by an approximately half radiusand to incident the signal light to the light receiving element 18,aberration due to the parabolic face (the reflecting surface) of theparabolic mirror 16 can be decreased and image blur can be reduced.

Furthermore, since such a constitution is employed that signal light iscondensed by the spherical lens 12 and the parabolic face of theparabolic mirror 16, it is made possible to reduce the refractive powerdue to the spherical lens 12 to reduce image blur due to the sphericalaberration.

Moreover, since use of three-axis alignment in the fixing process of thesecond cap member 13 b and the receptacle 2, an image of signal lightcan be accurately positioned to the light receiving face 18 a of thelight receiving element 18.

In addition, since the light receiving element 18 and thetrans-impedance amplifier 19 are mounted on the base 11 in proximity toeach other, it is made possible to improve a high frequencycharacteristic.

Further, by using the capacity of the ceramic chip type on the lightreceiving element substrate 17, it is made possible to prevent resonanceowing to inductances of wire bonds 70 a and 70 b connecting the lightreceiving element 18 and the trans-impedance amplifier 19.

Since the light passing-through hole 14 in which the spherical lens 12is inserted is in the cap member 13 and a sealed structure is realizedby inserting the spherical lens 12 into the light passing-through hole14, a reliable sealed structure can be realized inexpensively.

In this connection, in the first embodiment, the parabolic mirror isused as a reflecting mirror having a quadric surface reflecting surface,a hyperboloid mirror whose reflecting surface is a hyperboloid may beused. Further, in the first embodiment, though the photodiode is used asthe light receiving element 18, another photo-semiconductor element suchas an avalanche photodiode can be used.

Second Embodiment

A light receiving element module of the second embodiment will beexplained with reference to FIGS. 12A to 12C. In the light receivingelement module of the first embodiment, the photodiode 18 of the surfaceincident type is used as the light detecting element. In the lightreceiving element module of the second embodiment, a photodiode 180 of aback surface incident type is used so that a groove of the parabolicmirror 16 for connecting the light detecting element 180 and thetrans-impedance amplifier 19 via a wire bond becomes unnecessary. FIGS.12A to 12C are diagrams for explaining electrical connection of thelight detecting element 180 and the trans-impedance amplifier 19,wherein FIG. 12A is a vertical sectional view of the periphery of theparabolic mirror 16, FIG. 12B is a front view and FIG. 12C is a planview where the parabolic mirror 16 has been removed. In FIGS. 12A, 12B,and 12C, like reference numerals are designated to portions havingfunctions equivalent to those in FIGS. 10A, 10B, and 10C.

As illustrated in FIGS. 12A, 12B, and 12C, a light detecting element 180mounted on a light receiving element substrate 175 includes a photodiodeof a back face incident type having a light detecting face on a backface, and a light detecting face (a photodiode portion) 180 a is formedon the back face side. A pair of electrodes 175 a and 175 c (a pair ofp-side and n-side electrodes) are formed on surface side of the lightreceiving element substrate 175. A pair of unillustrated terminals (ananode and a cathode) of the light detecting element 180 are respectivelyconnected to the electrodes 175 a and 175 c of the light receivingelement substrate 175 by soldering. Further, a back surface electrode ofa capacitor 32 b is soldered on an upper face of the electrode 175 a. Asurface electrode of the capacitor 32 b is connected to anotherconductor pad 175 b of the light receiving element substrate 175. Theconductor pad 175 b is connected to a surface of the base 11 via athrough hole 175 e. One end of a wire bond 70 a is bonded to anotherconductor pad 175 d of the light receiving element substrate 175, andthe other end of the wire bond 70 a is connected to a pad 19 a of thetrans-impedance amplifier 19. The surface electrode of the capacitor 32b is also connected to a conductor pad 175 d of the light receivingelement substrate 175. One end of a wire bond 70 b is connected to anelectrode 175 c and the other end of the wire bond 70 b is bonded to apad 19 b of the trans-impedance amplifier 19.

According to the light receiving element module of the secondembodiment, since the photodiode of the back face incident type is usedas the light detecting element 180, it is made unnecessary to providethe groove 16 b (refer to FIGS. 10A to 10C) in the parabolic mirror 16for connecting the light detecting element 180 and the trans-impedanceamplifier 19 via a wire bond, so that working for the groove 16 b of theparabolic mirror 16 is made unnecessary and manufacturing cost of theparabolic mirror 16 can be reduced.

Third Embodiment

A light receiving element module of the third embodiment will beexplained with reference to FIG. 13. In the light receiving elementmodule of the first embodiment, the trans-impedance amplifier 19 isarranged on a rear stage side of the light receiving element 18 on thebase 11. In the light receiving element module of the third embodiment,the trans-impedance amplifier 19 is arranged on a front stage of thelight receiving element 18 on the base 11, so that space saving in awidthwise direction (a horizontal direction) of the light receivingelement module 3 is achieved. FIG. 13 schematically illustrates avertical sectional view of the light receiving element module 3 of FIG.1, where some portions are omitted and some portions are illustrated ina simplified manner. In FIG. 13, like reference numerals are designatedto portions having functions equivalent to those in FIG. 2. Asillustrated in FIG. 13, the trans-impedance amplifier 19 is arranged ona front stage side of the light receiving element 18 and the parabolicmirror 16 is arranged on a rear stage side of the light receivingelement 18. At this time, the parabolic mirror 16 is provided to thestrip differential data lines 31 a and 31 b such that the differentialmicrostrip lines 31 a and 31 b do not interfere with the light receivingelement 18.

According to the light receiving element module of the third embodiment,since the trans-impedance amplifier 19 is arranged on the front stageside of the light detecting element 18 on the base 11, it is madepossible to save space in the widthwise direction (a horizontaldirection) of the light receiving element module by the space for thetrans-impedance amplifier 19, as compared with the light receivingelement module of the first embodiment.

Furthermore, the wire bonds 70 a and 70 b for connecting thetrans-impedance amplifier 19 and the light receiving element 18 can bearranged ahead of the parabolic mirror 16 (on the side of the opticalfiber 20), and the trans-impedance amplifier 19 and the light receivingelement 18 can be connected to each other as shown in FIGS. 10A, 10B,and 10C.

Fourth Embodiment

A light receiving element module of the fourth embodiment will beexplained with reference to FIG. 14. In the light receiving elementmodule of the first embodiment, the sealed structure is formed byinserting the spherical lens 12 into the light passing-through holeformed in the cap member 13. In the light receiving element module ofthe fourth embodiment, a sealed structure is formed by arranging atransparent member in the light passing-through hole formed in the capmember 13. FIG. 14 schematically illustrates a vertical sectional viewof the light receiving element module of FIG. 1, where some portions areomitted and some portions are illustrated in a simplified manner. InFIG. 14, like reference numerals are designated to portions havingfunctions equivalent to those in FIG. 2.

A lens holding member 80 which holds the lens 12 is provided between thereceptacle 2 and the cap member 13. An end face of the lens holdingmember 80 is joined to one end face of the cap member 13 on the side ofa light passing-through hole 81 by welding or the like. Further, anouter periphery of the lens holding member 80 is fitted into an innerperiphery of a connection member 85 on its one end side, and theconnection member 85 is slid to the lens holding member 80 and weldedthereto. An end face of the connection member 85 on the other end iswelded to an end face of the receptacle 2 opposed to the hole 22thereof.

As illustrated in FIG. 14, a light passing-through hole 81 is formed inthe cap member 13, and the light passing-through hole 81 is covered witha transparent member (a window member) 82 which is formed of cover glassor the like and is fixed to an inner wall of the cap member 13 includingthe light passing-through hole 81 by a solder glass or the like. Asealed structure is achieved by the transparent member 82. The lensholding member 80 which has a cylindrical shape and, where a lightpassing-through hole for inserting the spherical lens 12 is located, isfixed to the cap member 13. The spherical lens 12 is inserted into thelight passing-through hole and fixed therein by adhesive or the like.Further, the receptacle 2 is fixed in the lens holding member 80.

According to the light receiving element module of the fourthembodiment, since the sealed structure is realized by fixing thetransparent member 82 to the inner wall side of the cap member 13 formedwith the light passing-through hole 81 to cover the lightpassing-through hole 81, the sealed structure can be realizedinexpensively and it is made possible to realize a reliable sealedstructure.

It should be noted that the present invention is not limited to theembodiments described above, and various modifications may be embodiedwithout changing the gist of the invention.

As explained above, according to the present invention, since the lightreceiving element module includes a lens which condenses signal lightemitted from an optical fiber, a reflecting mirror which has a quadricsurface reflecting surface reflecting the signal light condensed by thelens, and a light detecting element which detects the signal lightreflected by the reflecting mirror and converts the light into anelectrical signal, the region of the reflecting surface of thereflecting mirror can be made small and the reflecting mirror can bereduced in size. As a result, influence of a thermal expansioncoefficient due to material for the reflecting mirror can be reduced andstructure of the module can be simplified. Further, it is made possibleto provide an inexpensive light receiving element module which can bereduced in size.

INDUSTRIAL APPLICABILITY

As described above, the light receiving element module according to thepresent invention can be widely applied to a receiver and a transceiverfor an optical communication system using an optical fiber.

1. A light detecting element module comprising: a lens condensing signallight emitted from an optical fiber; a reflecting mirror having aquadric reflecting surface, an axis, and a center intersected by theaxis, the reflecting mirror reflecting the signal light condensed by thelens; and a light detecting element detecting the signal light reflectedby the reflecting mirror and converting the signal light into anelectrical signal, wherein the signal light condensed by the lens isincident on the quadric reflecting surface generally parallel to theaxis of the quadric reflecting surface, and the signal light incident onthe quadric reflecting surface within approximately one-half radius ofthe center of the quadric reflecting surface is reflected atapproximately a right angle to the axis of the quadric reflectingsurface.
 2. The light detecting element module according to claim 1,wherein a real images at an emitting point of the signal light in theoptical fibers is imaged by the lens to form a virtual image at a lightdetecting face of the light detecting element, on an optical axis of thelens, with the reflecting mirror.
 3. The light detecting element moduleaccording to claim 1, wherein the quadric reflecting surface isparabolic.
 4. The light detecting element module according to claim 1,wherein the quadric reflecting surface is a hyperboloid.
 5. The lightdetecting element module according to claim 1, wherein the lens is aspherical lens.
 6. The light detecting element module according to claim1, further comprising a trans-impedance amplifier located on a commonplane with the light detecting element, proximate the light detectingelements, and amplifying the electrical signal produced by the lightdetecting element.
 7. The light detecting element module according toclaim 1, wherein the reflecting mirror is formed using a plastic mold.8. The light detecting element module according to claim 1, includingadjusting the optical fiber along an optical axis direction and in twodirections perpendicular to the optical axis direction.
 9. The lightdetecting element module according to claim 1, wherein the lens has amagnification of at least one and no more than three, the reflectingmirror has a magnification of at least ⅙ and no more than one, andoverall magnification, including the lens and the reflecting mirror isat least 0.5 and no more than one.
 10. The light detecting elementmodule according to claim 1, wherein the reflecting mirror has a radiusof curvature and a focal length, one of which is no more than 1millimeter.
 11. The light detecting element module according to claim 6,further comprising a capacitor having a ground electrically connected toa ground of the trans-impedance amplifier, wherein the light detectingelement, the trans-impedance amplifier, and the capacitor are arrangedin substantially the same plane.
 12. The light detecting element moduleaccording to claim 6, further comprising: a base; and a capacitor onwhich the light detecting element is mounted and having a back faceconnected to a ground face of the base.
 13. A light detecting elementmodule comprising: a stem through which signal pins penetrate; a basefixed in a direction perpendicular to the stem; a cap member having alight passing through hole and fixed to the stem; a spherical lensinserted into the light passing through hole and condensing signal lightemitted from an optical fiber; a parabolic mirror located on the baseand reflecting the signal light condensed by the spherical lens atapproximately a right angle, wherein; the spherical lens has amagnification of at least one and no more than three, the parabolicmirror has a magnification of at least ⅙ and no more than one, andoverall magnification, including the spherical lens and the parabolicmirror is at least 0.5 and no more than one; a light detecting elementlocated on the base, receiving the signal light reflected by theparabolic mirror, and converting the signal light received into anelectrical signal; and a trans-impedance amplifier located on the baseproximate the light detecting element and amplifying the electricalsignal produced by the light detecting element.
 14. A light detectingelement module comprising: a stem through which signal pins penetrate; abase fixed in a direction perpendicular to the stem; a cap member havinga first light-passing through hole and fixed to the stem; a windowmember covering the first light passing through hole; a lens holdingmember having a second light passing through hole and fixed to the capmember; a spherical lens inserted into the second light passing throughhole and condensing signal light emitted from the optical fiber; aparabolic mirror located on the base and reflecting the signal lightcondensed by the spherical lens at approximately a right angle; a lightdetecting element located on the base, receiving the signal lightreflected by the parabolic mirror, and converting the signal lightreceived into an electrical signal; and a trans-impedance amplifierlocated on the base proximate the light detecting element and amplifyingthe electrical signal produced by the light detecting element, whereinthe spherical lens has a magnification of at least one and no more thanthee, the parabolic mirror has a magnification of at least ⅙ and no morethan one, and overall magnification. including the spherical lens andthe parabolic mirror is at least 0.5 and no more than one.